Methods for magnetic sensor having non-conductive die paddle

ABSTRACT

Methods for providing a sensor integrated circuit package including employing a conductive leadframe and forming a non-conductive die paddle in relation to the leadframe. The method can further include placing a die on the non-conductive die paddle to form an assembly, forming at least one electrical connection between the die and the leadframe, and overmolding the assembly to form an integrated circuit package.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is continuation of U.S. patent application Ser. No.15/049,732, filed on Feb. 22, 2016, which is a divisional application ofU.S. patent application Ser. No. 14/090,037 filed on Nov. 26, 2013, nowU.S. Pat. No. 9,299,915 entitled: METHODS AND APPARATUS FOR MAGNETICSENSOR HAVING NON-CONDUCTIVE DIE PADDLE, which is a continuation of U.S.patent application Ser. No. 13/350,970 filed on Jan. 16, 2012, now U.S.Pat. No. 8,629,539 entitled: METHODS AND APPARATUS FOR MAGNETIC SENSORHAVING NON-CONDUCTIVE DIE PADDLE, which is incorporated herein byreference in its entirety.

BACKGROUND

As is known in the art, eddy currents can degrade the performance ofintegrated circuits having magnetic sensors. Magnetic sensors typicallyinclude a magnetic transducer, such as a Hall cell element, on thesurface of an integrated circuit, which is mounted on a metal leadframe.The sensor is connected to the leadframe with wires and overmolded withthermoset plastic. While such magnetic sensors may be suitable forsensing static magnetic fields, at higher frequencies increasing eddycurrents are generated in the conductive leadframe in response to thechanging magnetic field. Eddy currents flow in circular loopsperpendicular to the direction of the magnetic flux vectors. The eddycurrents create an opposing magnetic field underneath the Hall cell,which can cause unacceptably large errors in the magnetic field strengthdetected by the sensor.

While prior art attempts have been made to provide slots in conductiveleadframes to reduce eddy current flow, such slots provide only limitedreductions in eddy current levels. U.S. Pat. No. 6,853,178 toHayat-Dawoodi, for example, shows various slots across the leadframe andcrossed slots.

SUMMARY

In one aspect of the invention, a method comprises: employing aconductive leadframe; forming a non-conductive die paddle in relation tothe leadframe; placing a die on the non-conductive die paddle to form anassembly; forming at least one electrical connection between the die andthe leadframe; and overmolding the assembly to form an integratedcircuit package.

The method can further include one or more of the following features:providing a non-conductive die paddle on which the die is disposed, thenon-conductive die paddle comprises a plastic material, a magnetic layeraligned with the die to affect magnetic fields proximate the die, aback-bias magnet as part of the IC package, the conductive leadfingermaterial is at least a given distance more than a height from theleadfingers to the magnetic sensing element, the conductive leadfingermaterial is at least two times a vertical height from the leadfingers tothe magnetic sensing element, the magnetic sensor element is formed inthe die, the magnetic sensor element includes a Hall element, themagnetic sensor element includes a magnetoresitive element, theleadfinger material extends from only one side of the magnetic fieldsensor device, and/or applying an underfill material proximate the waferbumps.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features of this invention, as well as the inventionitself, may be more fully understood from the following description ofthe drawings in which:

FIG. 1 is a graphical representation of propagation time for a prior artmagnetic integrated circuit;

FIG. 2 is a graphical representation of response time for a prior artmagnetic integrated circuit;

FIG. 3 is a graphical representation of rise time for a prior artmagnetic integrated circuit;

FIG. 4 is a prior art magnetic sensor IC package;

FIG. 5 is a prior art magnetic sensor IC package with a slot in aconductive leadframe;

FIG. 6 is a schematic depiction of a conductive leadframe that can forma part of an IC package having a non-conductive die paddle;

FIG. 7 is a schematic representation of partially fabricated IC packagein accordance with exemplary embodiments of the invention;

FIG. 7A is a side sectional view of the IC package of FIG. 7 without amagnetic layer;

FIG. 7B is a side sectional view of the IC package of FIG. 7 with amagnetic layer;

FIG. 8 is a schematic representation of a partially fabricated ICpackage in accordance with exemplary embodiments of the invention;

FIG. 8A is side sectional view of the assembly of the IC package of FIG.8 without a magnetic layer;

FIG. 8B is side sectional view of the assembly of the IC package of FIG.8 with a magnetic layer;

FIG. 8C is a side view of an assembly with a magnetic layer secured to aback of the non-conductive die paddle;

FIG. 8D is a side view of an assembly with a hard ferromagnetic materiallayer secured to the magnetic layer of FIG. 9C;

FIG. 9 is a schematic representation of an IC package in accordance withexemplary embodiments of the invention;

FIG. 9A is side sectional view of the assembly of the IC package of FIG.9 without a magnetic layer;

FIG. 9B is side sectional view of the assembly of the IC package of FIG.9 with a magnetic layer;

FIG. 10 is a flow diagram showing an exemplary sequence of steps forfabrication an IC package in accordance with exemplary embodiments ofthe invention; and

FIG. 11 is a schematic representation of an exemplary flip chipembodiment of an IC package in accordance with exemplary embodiments ofthe invention.

DETAILED DESCRIPTION

The present invention provides methods and apparatus for an integratedcircuit (IC) package including a die on a non-conductive die paddle toreduce eddy current effects on a magnetic sensor. In one embodiment, aSingle In-line Package (SIP) with a non-conductive or high resistivityplastic die paddle allows design flexibility and improved magneticsensor performance when encapsulating magnetic semiconductor IntegratedCircuits (ICs). The non-conductive or high resistivity is large enoughsuch that an eddy current that results in an unacceptably large magneticfield error is not induced in the application. The non-conductive diepaddle improves the response time and bandwidth of magnetic sensors forhigh frequency applications, such as DC-DC converters and inverters inswitch mode power supplies. In an exemplary embodiment, a layer offerromagnetic or magnet material is placed inside the package. Theferromagnetic or magnetic material may be either a soft ferromagnetic ora hard ferromagnetic material, or in some cases both a soft and hardferromagnetic material layer and multilayer. It is understood that theterm “die paddle” refers to the area of the leadframe or package that adie or multiple die may locate in the final package construction.

Before describing exemplary embodiments of the invention, someinformation is provided. Magnetic sensor integrated circuits, whichcontain transducers, including but not limited to, Hall Effect, MR(magnetoresistive), GMR (giant magnetoresistive, AMR (anisotrpicmagnetoresistive) and TMR (tunneling magnetoresistive) type devices haveinherent bandwidth limitations due to the physical and electrical designof the Integrated Circuit (IC). Magnetic sensor circuits have inherentcapacitance, inductance, and resistance that form some type of tunedcircuit determining the overall frequency response/bandwidth of thetransducer circuit on the magnetic IC. This bandwidth is typicallyrelatively high, e.g., from about 50 Hz to hundreds of kHz for sensoroutput. This bandwidth is often filtered on the IC in amplification andfiltering stages to optimize device performance and lower output noise.It is understood that filtering can be minimized, usually at the expenseof accuracy. With a high bandwidth design, the physical packaging shouldbe considered because it will limit the response time for high frequencymagnetic events, as discussed below.

FIG. 1 shows the propagation delay (t_(PROP)) of a conventional magneticintegrated circuit. The propagation delay is the time required for themagnetic sensor output to reflect a change in the applied magneticfield. Propagation delay is attributed to magnetic transducer signalconditioning in the IC and to inductive loading within the linear ICmagnetic sensor package, as well as the inductive loop formed by theprimary conductor geometry creating the magnetic field.

FIG. 2 shows the device response time (t_(RESPONSE)), which is definedas the time interval between when the applied magnetic field reaches 90%of its final value and when the magnetic sensor IC output reaches 90% ofits output value corresponding to the applied magnetic field.

FIG. 3 shows the device rise time (t_(r)), which is the time intervalbetween the magnetic sensor output reaching 10% of its full scale valueand reaching 90% of its full scale value. The rise time to a stepresponse is used to derive the approximate bandwidth of the magneticsensor, and is calculated as f(−3 dB)=0.35/t_(r). It should be notedthat the rise time t_(r) and response time t_(RESPONSE) aredetrimentally affected by eddy current losses observed in the conductiveIC die paddle, which is often also the ground plane. Therefore, thebandwidth and overall response time for a high frequency magnetic sensoris determined by the IC design, as well as the packaging.

In a conventional SIP configuration shown in FIG. 4, the die IC ismounted on the die paddle DP of the leadframe LF, which is oftenconnected to the GND lead of the package, shown as pin 4. The die IC isattached to the leadframe die paddle DP with a conductive adhesive andcontact from the die active areas to the leads is made with a gold wirebond WB. The assembly is then over-molded, for example with a moldcompound, to protect the die IC and wire bonds WB. Typically, manydevices are over-molded at the same time and singulated from the matrixleadframe after molding into individual units.

In conventional ICs, the leadframe material, e.g., plated copper, isconductive. The conductive leadframe LF allows eddy currents to formduring high frequency magnetic events. As is known in the art, eddycurrents are currents induced in conductors that oppose the change inmagnetic flux that generated the eddy currents. Eddy currents aregenerated when a conductor is exposed to a changing magnetic field dueto relative motion of the field source and conductor and/or fieldvariations over time. The resultant eddy currents create inducedmagnetic fields that oppose the change of the original magnetic fieldchange in accordance with Lenz's Law. The opposing field delays theresponse time of the magnetic sensor IC to reach the value of themeasured magnetic field. As seen in FIG. 4, the magnetic transducerelement MT is subject to both the incident and opposing magnetic fields.

FIG. 5 shows a prior art device having a portion of a copper leadframebehind the magnetic transducer removed to form a slot SL to reduce eddycurrent levels. While forming slots in a conductive leadframe may reduceeddy currents to acceptable levels, higher frequency operation may stillbe limited.

In one aspect of the invention, a magnetic sensor IC includes anon-conductive die paddle to minimize the amount of electricallyconductive material proximate the IC in order to reduce, if noteliminate, eddy currents. The die is attached to a non-conductivematerial, such as plastic, for example a non-conductive mold compound,instead of copper leadframe material. With this arrangement, eddycurrents near the integrated circuit are minimized, which concomitantlyminimizes the strength of the opposing field generated by the eddycurrents, and therefore, lowers the instantaneous error and reduces theresponse time.

FIG. 6 shows a leadframe 100 that can form the basis for an IC packagewith a non-conductive die paddle in accordance with exemplaryembodiments of the invention. Prior to formation of the non-conductivedie paddle, the leadframe 100 has only conductive portions 102. Theconductive portions 102 can be formed from copper or other metal toprovide lead fingers. In one embodiment, a Fe-Ni alloy, such as KOVAR(trademark of Carpenter Technology Corporation), is used. In general,the conductive leadframe material is outside a perimeter of the die. Thenonconductive die paddle to eliminate conductive material, e.g., copper,behind the sensor IC for reducing eddy currents can be formed asdescribed below.

FIG. 7 shows the assembly of FIG. 6 overmolded in a first mold step tocreate a non-conductive die paddle 200 with an optional magnetic layer202 in relation to a conductive leadframe 204. In other embodiments, themagnetic layer 202 can be provided as a ferromagnetic material that canbe used as a magnet or concentrator for magnetic fields behind a die.With a back biased ferromagnetic magnetic layer 202 in the plasticpackage, the bandwidth may be limited. However, in some applications aferromagnetic material, or a back biased magnet, may be more desirablethan high frequency operation. It should also be noted that thethickness of the magnetic layer is typically be less than that of theleadframe material. In the case of a back biased magnetic material theconductivity is lower, therefore resulting in lower eddy currents in themagnetic layer 202.

It is understood that the geometry and dimensions of the components inexemplary embodiments of the invention can vary to meet the needs of aparticular application. For example, die paddle materials can havedifferent lead thicknesses, which can vary depending on the packagedesign. Exemplary thicknesses include 8 mils, 10 mils, and 15 mils.However, packages such as MLP (micro leadframe) or QFN (quad flat noleads) may use less material, e.g., 5 mils. It is contemplated thatthickness will continue to decrease as technology improves, e.g., aspackage sizes and volumes continue to decrease.

In the illustrated embodiment, the conductive leadframe material 204does not overlap at all with the die. That is, where the die 206 islocated in a horizontal plane and the leadframe is located in the sameor different horizontal plane, no vertical line intersects both the dieand the leadframe. It should be noted that as long as any leadframeoverlap does not come near the magnetic field transducer the spirit ofthe invention is maintained.

The magnetic layer 202 can be provided in a wide range of geometries,dimensions and materials, to meet the needs of a particular application.In one embodiment, the magnetic layer is provided as a back biasedmagnet comprising, but not limited to: NeFeB, a hard ferrite, and/orSmCo. In other applications, the magnetic layer 202 is provided as asoft magnetic material when used to direct flux and a magnet is providedas a relatively hard magnetic material that applies flux. In the case ofa desire to isolate electrical influences, the magnetic layer may be aconductive layer, e.g., a ground plane.

FIG. 7A shows a side view of the assembly without a magnetic layer andFIG. 7B shows a side view of the assembly with the magnetic layer 202.It should be noted that while FIG. 7B shows the magnetic layer 204 flushwith the material 200, the material 202 may extend beyond edge or beshort of the edge of the material 200 for certain applications.

As shown in FIG. 8, once the first mold step to provide thenon-conductive die paddle 200 is complete, with or without the magneticlayer 202, a die 206 can be mounted on the plastic die paddle 200 andwire-bonded to create connections 208 from the die to the lead fingers.A magnetic transducer 210, such as a Hall element or magnetoresistor(giant magneotresistance (GMR), anisotropic magnetoresitive element(AMR), magnetic tunnel junction (MTJ), or tunneling magnetoresistor(TMR)), can be provided in the die in a manner well known in the art. Ingeneral, there is no overlap between the die 206 and the conductiveleadframe 204. It is understood that the spacing from the edge of thedie to any leadframe material would be considered by the designer for agiven application.

FIG. 8A shows a side view of the assembly without a magnetic layer andFIG. 8B shows a side view of the assembly with the magnetic layer 202.FIG. 8C shows a side view of an assembly with a magnetic layer 202secured to a back of the non-conductive die paddle 200. In anotherembodiment shown in FIG. 8D, a hard ferromagnetic material layer 205 canprovide a back-bias magnet instead of or in addition to the magneticlayer 202 provided by the soft ferromagnetic material.

As shown in FIG. 9, to complete the packaging a final overmold step withmold material 212 yields the final IC package. FIG. 9A shows a side viewof the assembly without a magnetic layer and FIG. 9B shows a side viewof the assembly with the magnetic layer 202. FIGS. 9A and 9B show anoptional dimple or reduced thickness of the package behind the die.

FIG. 10 shows an exemplary sequence of steps for fabricating an IChaving a non-conductive die paddle in accordance with exemplaryembodiments of the invention. In step 300, a leadframe is formed. In oneembodiment, the leadframe is fabricated from a conductive material, suchas copper, and is configured to provide leadfingers for the IC package.In step 302, a die paddle is fabricated from a non-conductive material,such as an electrically insulating, or non-conductive. plastic. In oneembodiment, the die paddle is formed using a mold process. The diepaddle is oriented with respect to the leadframe.

An optional magnetic layer can be provided in step 304. In oneembodiment, a magnetic concentrator or a permanent magnet is positionedin the die paddle as part of the die paddle molding process. Themagnetic material can be formed from a soft ferromagnetic material toprotect the die from magnetic fields behind the IC package. In anotherembodiment, a hard ferromagnetic material may be utilized to provide aback-bias magnet instead of or in addition to the magnetic layerprovided by the soft ferromagnetic material, as shown in FIG. 8D.

In step 306, a die is placed on the leadframe/die paddle assembly. Ingeneral, the die paddle is configured such that there is no conductivematerial overlapping or directly adjacent the die so as to reduce, ifnot eliminate, eddy currents proximate the die. In one embodiment, anadhesive, preferably, but not limited to, a non-conductive adhesive,secures the die to the die paddle. The die can include one or moremagnetic transducer elements. It is understood that eddy currents in anadhesive would be lower due the reduced thickness.

In step 308, wirebonds are formed between active areas of the die andlead fingers of the leadframe to provide the desired connections. Instep 310, the assembly can be overmolded to provide an IC package. Anysuitable overmolding material can be used.

FIG. 11 shows an exemplary flip chip configuration for an IC packagehaving a die 400 positioned on an optional non-conductive die paddle402. A magnetic transducer 403, such as a Hall element or amagnetoresistive element, can be provided in the die. Conductive leadfingers 404 have a bump area 406 to provide a connection to active areasof the die 400, which may have solder balls or stud bumps (for examplecopper pillars). The connection of the die to the leadframe is typicallyachieved via a reflow step. In an alternative embodiment, an epoxyadhesive is used at designated locations. An overmold material 408 isovermolded about the assembly to provide the IC package.

It is understood that a magnetic layer may also be used in conjunctionwith flip-chip embodiment. It is further understood that other methods,such as chip on lead technology, can also be used without departing fromthe scope of the invention.

In an exemplary flip chip embodiment, the die paddle step 306 andwirebond step 308 of FIG. 10, are modified to reflow bumps onto the leadfingers and apply an optional underfill material. In one embodiment,after place and reflow of the bumped die, the assembly is overmolded ina single molding step.

In one flip chip embodiment, conductive leadframe material is kept awayfrom the magnetic transducer, e.g., the Hall plate. A boundary region405 can define an area that contains no conductive material. In general,the boundary region 405 should minimize eddy current influences. In oneparticular embodiment, conductive leadframe material is at least 0.25mils away from a boundary of the Hall element. In another embodiment,the conductive leadframe material is at least two times the verticalheight from the leadframe to the transducers. In flip chipconfigurations, if the after reflow bump height is 50 to 75 microns, forexample, a distance of 100 to 200 um may be required. For wirebondedparts, this distance may need to be larger.

It is understood that the boundary region can comprise any suitablegeometry to meet the needs of a particular application. Exemplarygeometries include rectangular, circular, ovular, and other shapes thatenclose an area.

Exemplary embodiments of the invention provide a magnetic sensor ICcapable of increased frequency as compared to conventional sensors.Overmolding without an electrical or magnetic layer of conductive, softferromagnetic, or hard magnetic material in the first mold processproduces a package with minimal nearby copper leadframe material toconduct eddy currents. The packaged device is physically optimized forincreased frequency applications.

Using a layer of ferromagnetic material in the first overmold processlowers the bandwidth, but provides shielding from nearby interferingfields coming from the back side of the package for applications where asensor is looking for a field coming from one side of the package. Thislayer, in this case a magnetic concentrator layer, also concentrates orfocuses incident desired fields on the front of the package in caseswhere the field to be sensed is weak and allows for improved sensorperformance under weak field conditions.

Using a layer of hard or permanent magnetic material allows for anintegrated back biased magnetic solution to sense the motion of softferromagnetic material in front of the magnetic sensor IC. Thisback-biased magnet can be relatively thin, so that the generated fieldis relatively small. This configuration may be preferable formagneto-resistive solutions like GMR, AMR and TMR. This configurationcan be used in IC packages for gear tooth sensors, such as ABS(anti-lock braking systems) or transmission gear tooth sensors withrelatively small form factors. A thicker magnet allows for significantimprovement in the generated back biased magnetic field for Hall backbiased sensors which may result in increased working air gaps dependingon a particular magnetic design.

Having described exemplary embodiments of the invention, it will nowbecome apparent to one of ordinary skill in the art that otherembodiments incorporating their concepts may also be used. Theembodiments contained herein should not be limited to disclosedembodiments but rather should be limited only by the spirit and scope ofthe appended claims. All publications and references cited herein areexpressly incorporated herein by reference in their entirety.

What is claimed is:
 1. A method, comprising: employing a conductiveleadframe; forming a non-conductive die paddle in relation to theleadframe; placing a die on the non-conductive die paddle to form anassembly; forming at least one electrical connection between the die andthe leadframe; and overmolding the assembly to form an integratedcircuit package.
 2. The method according to claim 1, further includingcoupling a magnetic transducer to the die.
 3. The method according toclaim 1, further including forming the non-conductive die paddle from aplastic material.
 4. The method according to claim 3, further includingmolding the plastic die paddle.
 5. The method according to claim 4,wherein the leadframe does not overlap with the die.
 6. The methodaccording to claim 2, wherein the magnetic transducer does not overlapwith any of the leadframe.
 7. The method according to claim 2, furtherincluding molding the magnetic layer within the die paddle.
 8. Themethod according to claim 2, further including overmolding the diepaddle, the die, and the magnetic layer.
 9. The method according toclaim 1, further including positioning a Hall element in the die. 10.The method according to claim 1, further including positioning amagnetoresistive element.
 11. The method according to claim 1, furtherincluding a employing a non-conductive adhesive to secure the die to thedie-paddle, wherein a material comprising the non-conductive die-paddlesubstantially eliminates eddy current flow in the die paddle.
 12. Themethod according to claim 1, wherein a first surface of the leadframeand a first surface of the die are substantially coplanar.
 13. A method,comprising: employing a conductive leadframe; positioning anon-conductive die paddle in relation to the leadframe; positioning adie in relation to the die paddle; and coupling a magnetic transducer tothe die, wherein a cross section of the device through the magnetictransducer consists of non-conductive materials.
 14. A method,comprising: employing a conductive leadframe; positioning anon-conductive die paddle in relation to the leadframe, wherein the diepaddle comprises a plastic material; positioning a die in relation tothe die paddle; coupling a magnetic transducer to the die, wherein themagnetic transducer does not overlap with any conductive material; andforming at least one electrical connection from the die to theleadframe.